With conventional signal generators, a reference signal of variable frequency is supplied to a phase-locked oscillator. This is thus excited into oscillation at an adjustable frequency.
Accordingly, DE 41 05 566 A1 discloses a mixer, which synchronizes an oscillator to a clear frequency in the GHz range. This mixer operates with a multiplier diode, which generates a short pulse from the reference frequency, with the line spectrum of which the signal of the oscillator is mixed down. The disadvantage with this method is the poor signal-to-noise ratio which can be achieved with this arrangement. Since the sampling mixer converts on all harmonics of the reference signal, a large amount of noise is also mixed into the intermediate frequency. This restricts the sensitivity.
Moreover, US 2009/0309665 A1 discloses a high-frequency generator which contains a switchable phase-locked loop. Accordingly, the output signal of the oscillator is supplied optionally to a frequency splitter or a series circuit of several mixers. The signal of the frequency splitter is used to adjust the oscillator to a coarse frequency. In order to implement a fine adjustment, the arrangement then switches to the signal of the series-connected mixers. Even with a signal generator according to this design, it is only possible to achieve a sub-optimal phase noise.
Accordingly, there is a need for a high-frequency signal generator, which achieves very good secondary-line spacing with low phase noise.